Electrostatic coating of metal powder on metal strip

ABSTRACT

METHOD AND APPARATUS FOR APPLYING A METALLIC COATING ON METAL STRIP, PARTICULARLY STEEL STRIP, BY ELECTROSTATICALLY DEPOSITING ON THE STRIP A UNIFORM LAYER OF METAL POWDER AND THEREAFTER SUBJECTING THE STRIP AND THE LAYER OF METAL POWDER TO HEAT TREATMENT AND THEN COMPACTION TO EFFECT   COHESION OF THE PARTICLES OF METAL POWDER AND ADHESION OF THE METAL POWDER PARTICLES TO THE STRIP.

E. J. SMITH ET AL July 10, 1973 ELECTROSTATIC COATING OF ME Original Filed Aug. 15, 1967 INVENT( )RS' EDWIN J, SMETH ROBERT B. REIF ATTORNEY! United States Patent 3,745,034 ELECTROSTATEC COATRNG OF METAL POWDER ON METAL STEP? Edwin J. Smith, Winterville, and Robert- B. Reid, Grove City, Ohio, assignors to National Steel Corporation Continuation of application Ser. No. o6tl,787, Aug. 15, 1967. This appiication Aug. 14, U79, Ser. No. 63,896 int. (Ii. EilSh /02; 344d 1/094 US. Cl. l17-i7 8 Claims ABSTRACT OF THE DISCLOSURE Method and apparatus for applying a metallic coating on metal strip, particularly steel strip, by electrostatically depositing on the strip a uniform layer of metal powder and thereafter subjecting the strip and the layer of metal powder to heat treatment and then compaction to effect cohesion of the particles of metal powder and adhesion of the metal powder particles to the strip.

This is a continuation of application Ser. No. 660,787, filed Aug. 15, 1967, and now abandoned.

RELATED APPLICATION Application Ser. No. 534,420, filed Mar. 15, 1966 Leslie D. McGraw, Robert B. Reifl\z{ethod of Preparing Metal Coated Metallic Substrates and the Resultant Products, now Pat. No. 3,485,654.

BACKGROUND OF THE INVENTION This invention relates to methods and apparatus for applying a metal coating on metallic strip by electrostatically depositing metal powder on the strip.

In the past, metallic coatings have been applied to metal objects by the use of electrostatic spraying systems to deposit metal powder on the objects. Such systems employ a spray nozzle for discharging an aerosol of metal powder and a gas, such as air, and include high voltage electrodes placed in or near the nozzle to effect charging of the particles of metal powder. The discharge from the nozzle is directed toward the object to be coated which is maintained at a polarity opposite the polarity of the electrodes to establish an electrical field which moves the charged particles of metal powder toward and onto the surface of the object. Attempts to use such electrostatic spraying ssytems for applyin metallic coatings on metal strip have not produced a commercially acceptable process.

It is an object of the present invention to provide a novel method and apparatus for coating metal strip with metal powder by an electrostatic process.

Another object is to provide a method and apparatus of the foregoing character which overcomes the disadvantages of the prior systems and provides a commercially acceptable process.

Still another object of the present invention is to pro vide a novel method and apparatus for electrostatically coating metal strip with metal powder in a continuous process.

A still further object is to provide a novel method and apparatus which overcomes electrical shorting problems associated with electrostatic coating systems employing conductive particles.

Other objects and features of the invention will appear from the following detailed description considered in connection with the accompanying drawing which discloses one embodiment of the invention. It is to be expressly understood, however, that the drawing is designed for purposes of illustration only and not as a definition of the limits of the invention, reference for the latter purpose being had to the appended claims.

SUMMARY OF THE INVENTION Method and apparatus for applying metallic coating on metal strip by discharging a cloud of metal powder into a chamber through which the metal strip moves and in which the metal powder is electrostatically deposited on the metal strip, and method and apparatus for overcoming the problems of electrical shorting of the electrostatic deposition system attendant the electrostatic deposition of metal powder on metal strip.

BRIEF DESCRIPTION OF THE DRAWING In the drawing, in which similar reference characters denote similar elements throughout the several views:

FIG. 1 is a diagrammatic view in side elevation and partially in section of a coating apparatus provided by the present invention;

FIG. 2 is an enlarged fragmentary view, partially in section, of a portion of the apparatus shown in FIG. 1;

FIG. 3 is a view in section taken along the line 33 of FIG. 2; and

FIG. 4 is a three-dimensional view of the part of the apparatus shown in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT With reference more particularly to FIG. 1 of the drawings, a coating apparatus provided by the present invention is shown therein including a cylindrical drum 10 having an outer cylindrical metallic surface ill. The drum lit) is mounted on and rotates with a shaft 12 supported by suitable bearings, not shown, and, for a purpose that will be described below, the outer cylindrical surface 11 is maintained at ground potential such as by lead 13 connected to the shaft 12. The drum 10 is rotated in a clockwise direction by means of a variable speed motor 14 driveably connected to the drum by belt 15 and pulley in. Metallic strip 17 to be coated, such as steel strip, previously treated by any suitable conventional chemical process to present an impurity free surface, is delivered from an uncoiler l8 and guided by conventional guide rolls, not shown, for movement along a path including contact with a portion of the surface ll. of the drum iii and then through a furnace 19, a compaction mill 20 including rolls 2t and 22, and a furnace 23, to a coiling device 24. The coiling device 24 maintains the strip under tension in a conventional manner and the drum it) is rotated in the direction of strip travel and at such a speed that there exists slight, if any, relative movement between the strip 17 and the outer cylindrical surface ll of the drum.

The apparatus further includes a structure 25 forming a coating chamber 26 through which passes the strip material when in contact with the outer cylindrical surface l l of the drum 1t and wherein a surface of the strip receives a uniform coating of particles of metal powder in a manner described below. As shown more particularly in FIGS. 2 and 3 of the drawing, the structure 25 includes a housing having a top wall portion 27 and depending side wall portions 23- and 29 formed of nonelectrically conductive material. The top wall portion 27 overlies and is parallel to a segment of the surface ill extending throughout the width of the roller 1!) on which the strip 17 is supported and the free edges 30 and 31 of the side wall portions 28 and 29, respectively, are concavely curved to correspond to the curvature of the surface ll. for close sliding contact with opposite marginal side portions of the cylindrical surface 11. The structure 2.5 is supported on a stationary portion of the apparatus, not shown, to maintain the above-described relation with the drum and strip as shown in the drawing; detachable clamping means may be employed to permit movement of the structure away from the drum. The wall portions 27, 28 and 29 and the outer surface 11 of the drum 10, overlain by the top wall portion 27, define the coating chamber 26 which extends transversely of the drum between the side wall portions and angularly thereof throughout the portion of the cylindrical surface 11 in contact with the strip 17.

For a purpose that will be described below, the entrance opening 35 of the chamber 26 is substantially unrestricted, that is, of a cross-sectional area corresponding to the cross-sectional area of the chamber at any plane along its length, and the exit opening 36 communicates with a discharge chamber 37 defined by a housing of nonelectrically conducting material including top Wall 38 and lower wall 39. The inner transverse edge 40 of the wall 39 terminates in spaced parallel relation with the cylindrical surface I I to provide a slot 41 through which the strip passes on its way from the chamber 26.

A discharge nozzle 42 is located forward of the entrance opening 35 intermediate the sides of the cylindrical surface 11 and has a discharge opening 43 facing in a direction toward the entrance opening, the discharge opening being of substantially rectangular shape and positioned with its length dimension in spaced parallel relation to the surface 11. The nozzle 42 is connected by conduit 44 to the discharge of a blower 45. The blower may be of conventional construction having an air inlet, not shown, and a metal powder inlet 46 fed from a hopper 47 containing metal powder. Any suitable means such as an adjustable slitted plate-type valve 48 is provided for controlling the flow of metal powder to the blower. The blower delivers an aerosol, comprising particles of metal powder suspended in air, which is fed to the nozzle 42 and discharged from the opening 43 as a cloud of metal powder in a direction toward the entrance opening 35 and into the chamber 26. The discharge chamber 37 is connected to a source of suction 49 to induce the flow of powdered metal through the chamber.

A plurality of charging wires 50 are located in the chamber 26 in spaced relation relative to the direction of movement of the strip '17 and are each disposed transversely of the strip and parallel to the longitudinal axis of the drum 10 and in spaced relation with the inner surface 51 of the top wall portion 27 and the outer surface 52 of the strip 17. The charging wires 50 pass through suitable openings in the side wall portions 28 and 29 and, as shown in FIGS. 3 and 4, are supported outside the chamber 26 on opposite sides of the wall portions 28 and 29 by rollers 53 and 54, respectively. The charging wires beyond the rollers are connected to any suitable reeling and unreeling mechanism, not shown, for periodically moving different portions of the charging Wires through the chamber 26 for reasons that will be described below. The charging wires '50 are connected to a source 55 of high potential by common conductor 56 and individual conductors 57. If desired, the individual conductors 57 may be connected to respective charging wires by a connection to an insulated, conductive supporting shaft 58 for conductive rollers 54 as shown in FIG. 3.

As mentioned above, it is one of the objects of the present invention to provide an electrostatic coating method and apparatus including means which overcomes the electrical shorting problems attendant the electrostatic coating of metal strip with conductive powder. Such means includes the provision of the charging chamber 26 having an entrance opening 35 in combination with the metal powder discharge nozzle 42 including a discharge opening 43 of a cross-sectional area substantially less than the cross-sectional area of the entrance opening 35. This arrangement permits the flow of air around the discharge nozzle and through the entrance opening and into the chamber as indicated by arrows 60 of FIG. 2; the inflow of air being induced in part by the discharge from the nozzle and in part by the slight suction operating upon the discharge chamber 37. The flow of auxiliary air through the chamber 26 sweeps from the chamber particles of metal powder not deposited on the strip and minimizes the deposition of metal powder on the inside surfaces of the wall portions defining the chamber and prevents accumulation within the chamber of large masses of metal powder that could result in shorting the charging wires. The foregoing means also includes the feature of spacing the surface 52 and the edges of the strip from the inner surfaces of the structure 25 a distance greater than the distance between the charging wires 50 and the surface 52 of the strip. As seen more clearly in FIGS. 3 and 4, the transverse dimension of the strip 17 is substantially less than the transverse dimension of the chamber 26 and the strip is centered transversely of the chamber with bands 61 of non-conducting material positioned on the portions of the cylindrical surface 11 between the edges of the strip and the side wall portions 28 and 29. The chamber 26 extends in the direction of strip movement a distance sufficient to provide the deposition of the desired coating under given conditions of variables including for example strip speed and powder cloud density.

In operation, high potential from the charging source 55 is applied to the charging wires 50 to produce corona about the wires throughout their length within the chamber 26. The resulting intense ionization of gas molecules about the charging wires produces a large number of ions. The ions of a polarity opposite that of the charging source are attracted to the charging wires and neutralized while ions of the the same polarity as the charging source are repelled from the charging wires. The high potential applied to the charging wires also produces, since the strip 17 is maintained at the opposite polarity, an electrical field betweeen the charging wires and the surface 52 of the strip and the ions of the same polarity as that of the charging source are propelled by the electrical field in a direction toward the strip surface. At the same time, the blower 45 produces an aerosol of metal powder suspended in air which is discharged from the nozzle 42 as a cloud of metal powder particles which passes through the entrance opening 35 into the region of the chamber 26 between the charging wires 50 and the surface 52 of the strip; the particles of metal powder moving through the chamber 26 in a direction toward the exit opening 36 concurrently with the direction of movement of the strip. The particles of metal powder move through the path of the ions propelled from the charging wires and are charged by ion bombardment. Movement of the metal powder particles so charged is influenced by the electrical field and the charged particles within the field are accelerated in a direction toward and impinge upon and collect on the surface 52 of the strip. Such charging of the metal powder particles and their deposition on the surface of the strip by the electrical field occurs substantially throughout the chamber 26 and the strip leaves the chamber through the slot 41 uniformly coated with metal powder. Particles of metal powder which are not charged or which are otherwise not deposited on the surface of the strip are swept from the chamber 26 into the discharge chamber 37 due to the flow of air into the entrance opening 35 and through the chamber 26. Such action, as noted above, minimizes accumulations of metal powder on the inside surfaces defining the chamber 26 and precludes the formation of conductive paths which would effect shorting of the charging wires and interruption of the charging and deposition processes. Also, the charging wires 50 are continuously moved transversely of the chamber 26 to prevent the accumulation of metal powder on the charging wires. Such accumulation would increase the strength of the electrical field in the region of the accumulation and produce non-uniform deposition of metal powder on the surface of the strip. If desired, the discharge chamber 37 may be connected to a system for recovering undeposited metal powder and for returning the powder to the hopper 47. The metal strip having the coating of metal powder thereon leaves the charging chamber 26 through slot 41 and then passes through the furnace 19 to effect heat treatment of the strip and metal powder for efficient compaction in the compaction mill 20. The combined function of the furnace 19 and the compaction mill 20 effects cohesion between the particles of metal powder and adhesion between the metal particles and the surface of the strip, and the coated strip from the compaction mill 20 may be passed through furnace 23 for further heat treatment and then coiled on the coiling device 24 or fed to another line for further processing. If desired, the coated strip leaving the compaction mill 20 may not be subjected to further heat treatment in the furnace 23 prior to recoiling or further processing.

Apparatus embodying the principles of the present invention has been successfully operated to coat steel strip with aluminum. The steel strip was 1 mil thick and 4 inches wide and was centered on the drum between the side wall portions defining the coating chamber; the side wall portions were spaced by about 20 inches and the exposed side portions of the cylindrical surface of the drum were masked with Mylar. Four charging wires, comprising 0.005 inch diameter stainless steel wires, were mounted in the charging chamber transversely of the strip in the manner shown in the drawing. In operation of the apparatus, the following conditions existed:

Metal powder: Atomized aluminum having a mean mass particle size of microns Metal powder feed rate: 21-26 grams per minute Charging voltage: 13-15 kv.

Strip speed: 7.5 feet per minute Pro-compaction heat treatment: Temperature, about 850 F.; time, 1 to 2.5 minutes Compaction: Reduction rolling; about 0.8% elongation Post-compaction heat treatment: Temperature, about 950 F.; time, 0.5 to 2.5 minutes The strip leaving the coating chamber had deposited on its upper surface a uniform layer of aluminum powder and possessed good green strength. Following the precompaction heat treatment, compaction and post-compaction heat treatment, the aluminum coating on the steel strip was found to possess good coherence and adherence; and the thickness of the aluminum coating was about 0.1 mil. During operation of the apparatus, there was no appreciable accumulation of metal powder within the charging chamber and the operation was not inhibited by electrical shorting of the charging system.

It is to be understood that the gaseous carrier forming the aerosol of metal powder may comprise air, as described above, or an inert gas. Also, the auxiliary gas flowed into the entrance opening and through the charging chamber 26 may comprise air, as described above, or an inert gas. When the auxiliary gas comprises an inert gas, an inert gas supply duct would be attached to the opening 35 of the chamber 26 and the discharge nozzle 42 would pass through a wall of the supply duct with its discharge opening 43 located within the supply duct and positioned relative to the entrance opening of the chamber 26 generally in the manner shown in FIG. 2.

The novel features provided by the present invention which overcome the electrical shorting problems attendant the electrostatic deposition of metal powder on metal strip may be used in electrostatic coating apparatus operating in processes which may difier from the process disclosed herein and described above and may be used for applying metal coatings, in place of aluminum as described above, on strip material formed of other metals in addition to steel, as described above. For example, metal powder having a mean mass particle size greater than 5 microns may be used, such as commercially available metal powders of to 30 microns, and the strip before entering the coating chamber may be untreated or treated such as by wetting with water. Also, the temperature of the rare-compaction heat treatment may differ from the foregoing example and the post-compaction heat treatment need not be employed. Accordingly, the foregoing description including the specific examples are for purposes of description only and not as a definition of the limits of the invention, reference for the latter purpose being had to the appended claims.

We claim: 1. Method of applying a thin coating of metal on metallic strip which comprises the steps of continuously moving metallic strip through a confined zone having an entrance opening at one end and an exit opening at the other end and extending in the direction of strip movement, the strip moving in a direction through the zone from the entrance opening to the exit opening, intermixing metal powder and a gaseous carrier to form an aerosol and blowing the aerosol as a cloud of metal powder into the zone through the entrance opening generally in the direction of movement of the strip through the zone, charging metal powder by bombardment with ions within the zone, establishing an electric field within the zone to move charged metal powder within the zone in a direction toward and into contact with the strip to deposit metal powder on the strip, withdrawing excess metal powder at a point contiguous to the exit opening of the zone, passing strip moving from the exit opening of the zone to a heating zone, and passing strip from the heating zone between compacting rolls. 2. Method of applying a thin coating of metal on metallic strip as defined in claim 1 including the further Steps of introducing auxiliary of the zone,

and removing the auxiliary of the zone.

3. Method of applying a thin coating of metal on metallic strip as defined in claim 2 in which the carrier gas and the auxiliary gas is air.

4. Method of applying a coating of metal on metal strip by use of apparatus including non-conductive wall structure defining an elongated charging zone having an entrance opening at one end and an exit opening at the other end and a plurality of charging wires spaced longitudinally of the zone and disposed transversely of the zone, comprising the steps of continuously moving metal strip through the zone in a a direction from the entrance opening to the exit opening,

maintaining the path of movement of the metal strip through the zone spaced from the wall structure and the charging wires with the space between the metal strip and the charging wires being less than the space between the strip and the wall structure,

intermixing metal powder and a gaseous carrier to form an aerosol and blowing the aerosol as a cloud of metal powder through the entrance opening into the zone between the strip and the charging wires generally in the direction of movement of the strip through the zone,

applying an electrical potential to the charging wires to form corona about the charging wires producing ions which charge the particles of metal powder within the zone and establishing an electrical field between the charging wires and the metal strip to move charged particles of metal powder in a direction toward and into contact with the strip to deposit metal powder on the strip,

gas into the entrance opening gas from the exit opening withdrawing excess metal powder at a point contiguous to the exit opening of the zone, passing strip moving from the exit opening of the zone to a heating zone, and passing strip from the heating zone between compacting rolls. 5. Method of applying a coating of metal on metal strip as defined in claim 4 including the further steps of introducing auxiliary gas into the entrance opening of the zone, and removing auxiliary gas so introduced into the zone through the exit opening of the zone. 6. Method of applying a coating of metal on metal strip as defined in claim 5 in which the carrier gas is air.

7. A method of applying a coating of metal on a metallic substrate which comprises the steps of continuously moving the metallic substrate through a coating zone, inter-mixing metal powder and a gaseous carrier to form an aerosol including metal powder particles, introducing the aerosol of metal powder particles into the coating zone, charging the metal powder particles b-y bombardment with ions whereby electrically charged metal powder particles are present in the aerosol within the coating zone, establishing an electric field to move electrically charged metal powder particles within the coating zone in a direction toward and into contact with the substrate and to deposit a coating of metal powder on the substrate, passing the metal powder coated substrate from the coating zone to a heating zone and heating the substrate therein, and passing the substrate from the heating zone between compacting rolls and compacting the metal powder thereon. 8. A method of applying a coating of metal powder particles on a metallic strip which comprises the steps of passing a metallic strip through an electrostatic coating zone,

admixing metal powder particles and a gaseous carrier to form an aerosol including the metal powder particles,

introducing the aerosol of metal powder particles into the electrostatic coating zone,

electrically charging the metal powder particles while suspended in the aerosol by bombardment with ions whereby electrically charged metal powder particles are present in the aerosol within the electrostatic coating zone, and

establishing an electric field to move the electrically charged metal powder particles within the electrostatic coating zone in a direction toward and into contact with the metallic strip to deposit a coating of the metal powder particles thereon.

References Cited UNITED STATES PATENTS 3,000,752 9/ 1961 Jackson et al. 117-17 2,551,035 5/1951 Miller 117-17 3,186,860 6/1965 Jones 117-17 3,382,085 5/1968 Wren et a1. 117-17 2,796,845 6/ 1957 Rendell .118-634 3,503,775 3/1970 Austin 117-17 3,485,654 12/1969 McGraw et al. 117-17 2,608,176 8/1952 Jenkins et al. 117-17 2,686,141 8/1'954 Sawyer 117-17 3,426,730 2/ 1969 Lawson et al. 117-17 3,453,134 7/1969 Haw 118-638 2,217,444 10/ 1940 Hill 117-17 MURRAY KATZ, Primary Examiner M. SOFOCLEO'U S, Assistant Examiner US. Cl. X.R.

117-22, 31, R, 131, 102, M, L, A; 118-627, 630, 308, 312 

